利用時間解析光譜研究鐵基硫族化合物超導體之超快動力學

Abstract

本論文利用超短脈衝雷射進行光學激發探測實驗，得到具有飛秒等級時間解析能力的時間解析光譜。在結合了低溫系統後，我們能研究鐵基硫族化合物超導體(iron chalcogenides)單晶在不同狀態下(超導態、正常態)準粒子間的交互作用是如何隨溫度變化。例如：傳統超導體是以聲子作為媒介使電子形成庫柏對(Cooper pair)，因此，釐清不同自由度(例如：電子、聲子、電子自旋)間的相互作用關係是研究超導機制的重要工作。 藉由時間解析光譜，我們得到硒化鐵(FeSe)與摻碲硒化鐵(Fe1.05Se0.2Te0.8)晶體中電子-聲子耦合常數分別為0.16和0.01。如果硒化鐵與摻碲硒化鐵是傳統超導體，則以McMillan公式估算的超導相變溫度皆應低於1 K，但同一實驗下的硒化鐵樣品其實際相變溫度為8.8 K，而摻碲硒化鐵甚至具有稍高的相變溫度(10 K)。然而，藉由分析時間解析光譜的振盪特徵，我們在晶格相變溫度與超導相變溫度皆觀察到聲子軟化的現象。因此可以得知，即使超導相變與聲子相關，但鐵硒系列的鐵基超導體並非傳統超導體，至少電子-聲子耦合作用並非唯一促成電子配對的機制。 接著，我們利用具有軸向解析的超快光譜研究硒化鐵單晶的電子向列性(nematicity)與磁漲落(magnetic fluctuation)。除了成功觀察到費米面的電子結構在晶格相變溫度(約90 K)以下具有二重對稱性，當溫度超過晶格相變溫度而使得晶格對稱性由二重對稱轉變為四重對稱，即便高達200 K時，我們仍可清楚觀察到二重對稱的電子向列性漲落(nematic fluctuation)。由於此溫度遠高於晶格相變溫度，可以推論晶格相變並不是電子向列性的成因。此外，透過變溫實驗與分析，我們彙整出電子向列性與磁漲落隨溫度變化的相圖。 最後，同樣是鐵基硫族化合物超導體的鹼金屬鐵硒基超導體(AxFe2-ySe2)，由於在晶體中具有空間上分離的金屬相與反鐵磁絕緣相，晶體中準粒子的交互作用會更為複雜。因此，雙波長超快光譜提供了一個從時間維度上解析鹼金屬鐵硒基超導體中兩個相的相互關係以及準粒子的動力行為。從瞬時反射率光譜中，我們得到鹼金屬鐵硒基超導體單晶的超導能隙大小約為10.8 meV，並且可以估算超導相在20 K時約佔總體積的16%以及室溫下晶體的聲速為2.98 km/s。此外，變溫瞬時反射率光譜除了標示出約30 K的超導相變溫度，也在230 K清楚觀察到軌道選擇莫特相變(orbital selective Mott phase transition)。從電子弛緩時間在100 K發生的奇異轉折，我們得知金屬相與反鐵磁絕緣相之間在100 K以下透過電子交互作用與兩相之間的金屬界面相建立能量交換通道。

We utilized femtosecond time-resolved pump-probe spectroscopy to study the quasiparticle dynamics and the correlations between different degrees of freedom (such as charge, phonon, or spin) in single crystals of superconducting iron chalcogenide. We combined an optical pump-probe technique and a cooling system to study the temperature-dependent quasiparticle dynamics of iron chalcogenides in their superconducting state and in their normal state. Investigating the correlations between different degrees of freedom is a key topic for clarifying the mechanism of iron-based superconductivity. We obtained the electron-phonon coupling constants of FeSe and Fe1.05Se0.2Te0.8 from time-resolved spectroscopy. The coupling constants are 0.16 and 0.01 for FeSe and Fe1.05Se0.2Te0.8, respectively. If Fe1+ySe1-xTex is a conventional superconductor, the McMillan formula should estimate the superconducting transition temperature to be lower than 1 K. However, the superconducting transition temperatures of FeSe and Fe1.05Se0.2Te0.8 samples are 8.8 K and 10 K, respectively. In addition, phonon softening can be observed not only at structural transition temperature but also at superconducting transition temperature. These results imply that Fe1+ySe1-xTex is an unconventional superconductor and a phonon-mediated process cannot be the only mechanism leading to the formation of superconducting pairs in iron-based superconductors. Furthermore, by using polarization- and temperature-dependent time-resolved spectroscopy, we have successfully observed two-fold symmetry characteristics on the transient reflectivity of FeSe single crystals below the structural transition temperature (~90 K), which were attributed to electronic nematicity and magnetic fluctuation. Nevertheless, we were able to observe the two-fold symmetry characteristics from nematic fluctuation till 200 K even though the crystal structure transited from orthorhombic to tetragonal when the temperature was higher than structural transition temperature. Accordingly, the structural transition cannot be the origin of electronic nematicity. Moreover, we present a phase diagram of the temperature-dependent nematic order and the magnetic fluctuations of FeSe single crystals. Finally, the interactions between quasiparticles in alkali metal intercalated iron selenides are notably complicated due to the coexistence of spatially separated metallic phase and antiferromagnetic insulating phase. The transient reflectivity of (Na0.32K0.68)0.95Fe1.75Se2 single crystals under dual-color pump-probe spectroscopy showed that the superconducting gap is 10.8 meV. The volume fraction of the superconducting phase at 20 K and the sound velocity were estimated to be 16% and 2.98 km/s, respectively. In addition, the temperature-dependent transient spectroscopy clearly showed not only the superconducting transition at ~30 K but also the orbital-selective Mott phase transition at 230 K. The anomalous change of electron relaxation time at 100 K implies both the electronic correlation and the metallic interface phase between the two main phases serve as energy transfer channels for these two separated phases.